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Gem Under the Mountains
How did a gray speckled rock turn into the beautiful Gore Mountain Garnet, with minerals that have grown to be the size of a football? Imagine the heat of an erupting volcano or the weight of a mountain. In other words, it takes extreme heat and tremendous pressure. The Gore Mountain Garnet was once a gabbro, a dark igneous rock with tiny crystals. Over time, the Earth's crust slowly moved, and the gabbro was squeezed, crushed, and pushed deep underground. This movement of the crust formed the Adirondack Mountains in northern New York, where the gabbro was buried under Gore Mountain. The intense heat and pressure broke down the minerals in the gabbro to form black hornblende and red garnet. As this was happening, fluids seeped into the rock, which helped the garnet grow especially large. The gabbro's color, texture, and minerals completely changed to form the Gore Mountain Garnet.
Definition: rock that changes from heat and pressure
Where it forms: usually deep within the Earth's crust where it is hot and pressures are high
Common examples: marble, slate, schist
Cool fact: The mantle is made entirely of metamorphic rock.
The word "metamorphic" comes from the Greek word morphe, which means "form," and meta, meaning:
rock
change
minerals
Correct!
Metamorphic means "to change form"-which is exactly what metamorphic rocks do. The heat and pressure change everything: their texture, color, grain size, and the shapes and even the types of their crystals.
Metamorphic rocks are likely to be found:
at the bottom of rivers
on the beach
deep under the Earth's mountains
Correct!
Metamorphic rocks are likely to be found deep under the Earth's mountains. That is where rocks are crushed, folded, and squeezed by the movement of the Earth's tectonic plates.
Metamorphic rocks can only form from sedimentary and igneous rocks.
Fiction
Metamorphic rocks can also form from other metamorphic rocks. In the rock cycle, every kind of rock can be turned into another kind of rock.
A metamorphic rock has gone through so much heat and pressure, it is usually weaker than the original rock it formed from.
Fiction
The heat and pressure actually make the rock stronger. When limestone becomes the metamorphic rock marble, it becomes much more durable.
Metamorphic rocks are the most beautiful rocks you can imagine. They have swirling layers and large, beautiful minerals in all colors of the rainbow!
Rondi Davies, Earth scientist